There has been considerable recent interest in the incorporation of nanoscale components in lab-on-a-chip fluidic devices. This interest owes its origin to several advantages (and differences that may be advantageously leveraged) in moving from the micron scale to the nanoscale. These differences can include, for example, one or more of double-layer overlap (DLO) and its effect on electro-osmosis and charge permselectivity, localized enhancement of electric fields, higher surface to volume ratios, confinement effects on large synthetic and biopolymers, and the emerging importance of entropic effects. See, e.g., Yuan et al., Electrophoresis 2007, 28, 595-610; Schoch et al., Rev. Mod. Phys. 2008, 80, 839-883; and Kovarik et al., Anal. Chem. 2009, 81, 7133-7140.
Nanochannels are well suited for a number of applications including single molecule detection and identification, confinement and manipulation of biopolymers, biological assays, restriction mapping of polynucleotides, DNA sizing, physical methods of genomic sequencing, and fundamental studies of the physics of confinement. See, e.g., Riehn et al., Restriction mapping in nanofluidic devices. Proc. Natl. Acad. Sci. USA 2005, 102, 10012; Reccius et al., Conformation, length, and speed measurements of electrodynamically stretched DNA in nanochannels. Biophys. J. 2008, 95, 273; and Cipriany et al., Single molecule epigenetic analysis in a nanofluidic channel. Anal. Chem. 2010, 82, 2480. It is expected that the successful implementation of at least some of the potential applications will require the careful control of molecular dynamics within the nanochannels, including the velocity of molecular transport and the frequency with which analyte molecules are driven through the nanochannels. The transport of a macromolecule from macroscopic and microscopic reservoirs through nanofluidic conduits that are smaller than the molecule's radius of gyration may require the application of a driving force (e.g., hydrodynamic, electrostatic, gravitational) to overcome an energy barrier. This barrier is primarily entropic in nature and can derive from the reduction in the molecule's conformational degrees of freedom in moving from free solution to the confining nanochannel. See, Brochard et al., Dynamics of confined polymer chains. J. Chem. Phys. 1977, 67, 52, Additionally, the probability of a successful transport event can be proportional to the likelihood that the molecule collides with the entrance of the nanofluidic conduit in a conformation favorable to threading. See, Kumar et al., Origin of translocation barriers for polyelectrolyte chains. J. Chem. Phys. 2009, 131, 194903. The practical implication of these fundamental conditions is that molecular transport does not occur until a finite threshold driving force is applied. The magnitude of the requisite force may be considerable, resulting in transport of the analyte through the nanochannel at high velocity. The energy barrier can preclude or inhibit successful transport of the analyte molecule at lower velocity through a nanochannel, such lower analyte velocities may be desirable for many applications.